(+)-syn-Benzotriborneol: the first functionalised enantiopure C3-symmetric benzocyclotrimer

Copper(I) thiophencarboxylate readily and effectively cyclotrimerises protected 5-bromo-6-trimethylstannylborn-5-en-2-ols, leading to oxygen-functionalised benzocyclotrimers. The remarkably high syn to anti ratio is driven by relative displacement of the halide and metal on the alkene moiety and depends on the steric hindrance of the protecting group of the hydroxy function. The less hindered methyl derivative affords exclusively syn-benzocyclotrimer. The readily removable methoxymethyl derivative gives enantiopure syn-tribenzoborneol in high yields.